1. Filed of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method. In particular, the invention relates to a lithographic apparatus comprising an improved illumination system.
2. Description of the Related Art
Generally, an illumination system is a system for providing a beam of radiation, in order to illuminate an object. Said illumination may be desired for instance in order to be able to view the object, to expose the object to said radiation in order to start or maintain a chemical reaction, and so on.
A known instance of such an illumination system is a slide projector. In a slide projector, as in other illumination systems, it is important that the quality of the beam of radiation is high. For example, for many application, the intensity of the beam should be equal in every part of the beam cross-section. This requirement of homogeneous intensity over all of the beam may be even stricter for an illumination system in special applications, in particular a lithographic apparatus. Lithographic apparatus as intended in the present application will be elucidated hereinbelow. In a lithographic apparatus, radiation is used to impart patterns to specially prepared substrates. The substrates are made light sensitive. Features on the substrate, such as dimensions and definition thereof, are highly dependent of a.o. the beam being as homogeneous as possible over all of its cross-section.
For the purpose of the present invention, the words ‘beam’, ‘projection beam’ and ‘beam of radiation’ are in principle interchangeable terms. Note, however, that in some cases ‘projection beam’ is used specifically for a beam that will illuminate a patterning device, and a ‘patterned projection beam’ is used for a beam that has been patterned by striking a patterning device. The beam of radiation generated by a source (and subsequently collimated by a collector) that is being conditioned in the illumination system (illuminator) is sometimes referred to as the intermediate beam or just as the beam of radiation. The projection beam obviously has a direct relationship to this intermediate beam of radiation. The invention relates to improving the homogeneity of the intensity of a beam in general, whether or not specific patterning devices are used, but whenever a distinction is to be made between a beam in general and a (patterned) projection beam, this will be clear from the context. It is particularly important that the projection beam is as homogeneous as possible, since after patterning the possibilities to correct uniformity without affecting the pattern are limited.
Existing illumination systems, in particular for lithographic apparatus, are not always satisfactory as to the requirement of having a sufficiently homogeneous intensity of the projection beam of radiation. This is sometimes referred to as a sufficient uniformity.
In particular, present lithographic scanners still suffer from high spatial frequency non-uniformities, after low spatial frequency non-uniformities have been corrected. High spatial frequency modulations in intensity are not acceptable in, for example, the production of CCD (Charge Coupled Device). In addition, high spatial frequency non-uniformities may lead to increased levels of LER (Line Edge Roughness).